In a typical blast furnace operation, several stoves act to preheat the air that is blown into the blast furnace. This air is heated in the stoves by burning combustible carbon monoxide gas which is a by-product gas resulting from the interaction of carbon dioxide and coke in the furnace operations. Air for the combustion of the carbon monoxide is provided by large fans called burner fans. These fans are operated by an electrical energy source but the air intake on the fans is regulated by hydraulically-powered louvres. Thus, a power failure or a hydraulic failure will interfere with the proper functioning of the fans. In the event the flow of air from these fans to the stove is interrupted due to a power or a hydraulic failure, it is important that no carbon monoxide gas leak into the blast furnace stove area from the gas supply line connecting the furnace to the stove, since leaking carbon monoxide gas presents a safety hazard. In normal blast furnace operations, the potential exists for large amounts of carbon monoxide gas to leak into an area where it can pose a serious threat to human life.
A blast furnace stove usually has two valves which together act as a safety device to prevent the leakage of carbon monoxide gas. One of the valves operates as a gas shutoff valve and controls the flow of gas from the gas supply line to an area known as the gas burner passageway near the stove. This valve is ordinarily a tight seal butterfly valve and is often actuated by a mechanical device such as a motor. A tight seal valve is one which has a seal around the valve seat. The second valve operates as a combination gas regulating valve and safety valve. As its name implies, this valve regulates the amount of gas fed into the blast furnace stove while also performing a safety function, as described below. This valve is usually a clearance-type butterfly valve. Unlike a tight seal valve, the design of a clearance-type valve is such that when the valve is in a closed position, there is still a small amount of space, e.g. 1/16 of an inch between the valve disc and valve seat. This design feature results in a 1-2% leakage when the valve is closed.
The stove valve system operation is ordinarily designed so that the gas regulating valve can be driven to the open position only after the burner fan is operating and the gas shutoff valve is opened. When the gas shutoff valve is closed, the gas regulating valve is also driven to the closed position. This design allows the gas regulating valve to act as a safety device also.
One problem with such valve system, however, is that in the event of a power or a hydraulic failure, the gas shutoff valve, which is usually mechanically activated, must be manually closed. Manual closure is difficult because the gas shutoff valve is a tight seal butterfly valve which requires a large amount of force in order to be closed. Furthermore, manual closure is hazardous because if the gas regulating and safety valve does not fail-closed, carbon monoxide will leak into an area where a worker must enter in order to close the gas shutoff valve. By fail-closed is meant complete closure of a valve disc in the event of a power or a hydraulic failure. Clearance-type valves such as the gas regulating and safety valve, unlike tight seal butterfly valves, require less force for closure, but their operating mechanisms will not always cause these valves to fail-closed in the event of a power or a hydraulic failure. In addition, even when such clearance-type valves do fail-closed, their design is such that there is a 1-2% leakage, as mentioned above. Thus, substituting a clearance-type valve for a tight seal valve has not completely solved problems relating to the fail-closing of blast furnace stove valves.
In an attempt to eliminate manual closure of valves, devices known as magnetic clutches, which are operated by an external energy source, have been used in conjunction with various actuators. Under normal operating conditions, the clutch is energized allowing the valve to be in the open position. In the event of a power failure, the clutch is de-energized allowing the valve to close. The use of such clutches, however, has not eliminated problems relating to the fail-closing of valves, i.e., some valves still do not fail-closed even with such clutches. Actuators operated by external energy sources have also been employed in valve systems but these actuators have not always been successful in eliminating problems relating to the fail-closing of valves. Furthermore, valve systems employing various types of actuators have resulted in complex valve systems. Because they have a substantial number of parts, complex valve systems are expensive and problems with reliablity occur more often because of an increase in the frequency of part breakdown.